Fuel starvation?

[007brendan]

Let me just describe the symptom and tell me if it sounds familiar to anyone.

My commute to work is just slightly uphill, usually with a fairly strong headwind.  About the time I hit 65mph, my bike starts to jump and sputter likes it's running out of gas.  I've noticed this the last two times i drove to work.  I can't be sure, but sometimes it felt as though the problem went away if i cranked the throttle open.

On the way home, mostly downhill, a few uphill sections, but this side is largely protected from the wind. I don't get the jumping and sputtering on the way home.

I've done two recent changes, exhaust/lunchbox/rejet (40/150/3turns/1washer), and I switched out the rear taillight and turn signals for an integrated led taillight.
My gas tank is full.
I'm running with the fuel petcock set to prime.

I did the exhaust, etc. about a month ago and haven't noticed any problems, though, I haven't done a whole lot of highway riding in the past month.  The led change was last night.

[Anaconda]

I actually felt that similar situation this past weekend when i went out for a 50 mile ride, where i would see my revs drop as well.... would love to hear some advice as well.

[offcamber]

What year is you bike? If you bike uses the vacuum actuated petcock this may be your problem. I run into a similar problem cruising at  70-75 for more than 5-10 minutes. Try running it in prime and see if that fixes the issue.

[pandymai]

check the routing of your lines to see if any overflow and vacuum lines are pointing out? not sure but it's been said at least once in the past month or so, the overflow tubes help vent the tank and sometimes get pointed into the wind, causing there to be some suffocation of the fuel lines. something like that. sorry im not much help

[alyndaker]

I don't want to thread-jack, but I'm having something that sounds somewhat similar with my 2001 GS500 - here's the story:

Symptoms:

-The bike will start easily and run well standing still.  I can open the throttle and hold it at any engine speed I like.
-I can drive hard, consistently near or at wide open throttle and it responds well. 
-At low speeds in first through third, the bike behaves fairly normally.
-When above around 40mph in appropriate gears for general riding somewhere around 1/4 throttle, the bike acts like it is starving for fuel.  The misses last for between 0.5s and 3s before it hits for a few cycles then misses again.
-At any time during the poor behavior, I can open the throttle and it behaves normally.  I can also slow down and downshift and it will behave fine.

Things I have tried:
-Fuel - I initially drained the tank from the fuel line entering the carburetors since the previous owner was kind enough to block off the main drain.  I found no water here.  More recently, while being frustrated with other parts and checking the fuel shut-off valve, I spilled gas and saw water bead up.  After draining the fuel the hard way, I recovered about 1l(!) of water.  With the entire fuel system dry (carburetors included since they were disassembled at the time), I refueled and saw absolutely no change in performance. 

-Carburetors - I have fully disassembled and cleaned them twice with no change in the running characteristics.  All jets and internal passages are spotless.  The rubber boots on both sides appear to be pliable and leak free.  The vent hose is clear.  Using a piece of clear tubing, I have checked the still float heights and found them both to be slightly above the gasket.  While I don't know which jets are installed, I do know that they did work fine for the past two years and for the first few rides this year.  The slide diaphragms are soft, pliable, and leak free.  The jet needles show no signs of wear.  I toyed with the fuel screws going from too rich to too lean and saw no change in this behavior other than generally what one would expect with those conditions.

-Petcock - I have disassembled this and found it to be functioning well.  The vacuum line and diaphragm are leak tight and operating.  All three positions operate as they should.

-Fuel Lines - I initially rerouted the lines to remove the petcock from the circuit as well as all of the air space in the line.  I also replaced them with a more appropriate material than that which the former owner chose.  They are currently installed and routed as designed.

-Fuel Shut-off Valve -I pulled this and the screen assembly and found no obstructions.  The screens are clear with the exception of mild sediment as expected.  If one opens the valve, fuel will pour from both the primary and reserve take-off points.

-Fuel Tank - The vent is likely working due to the rate at which fuel flows from the ports.  To verify, I drove with the gas cap open and saw no change.

-Valve Clearances - Three valves are in spec and one of the intake valves is only marginally out of spec on the tight side. 

-Battery - I forgot to remove the battery last winter and consequently had to replace it this spring when I found that it would no longer hold a charge.  Whether correlated or not, this is about the time the misfiring problem got far worse.  The voltage across the battery when the bike is off is around 12V and at high RPM, it settles to about 14ish.

-Spark Plugs - I have replaced them as a long shot with no effect.  A running-kill-check test shows that they are slightly on the lean side but still seem fairly normal.  There is a strong spark from both plugs when I check.

Because nothing I have tried has even changed the behavior, I'm leaning toward the problem actually being in the ignition somewhere though I don't know where to go next.  Does anyone have any suggestions?